Dear Tom,
I used to work at Adobe [twice!] and the there's still quite a
historical legacy from the good ol' days of "FrameViewer", and when
FrameMaker was available on Macintosh, NeXT, SunOS, IBM AIX and more.
The "Hypertext" system is one of those legacies, and it probably
outdates the internet "hypertext" system.

As a result most [if not all] of the "Open document" functionality
should refer to FrameMaker documents, which if you print to PDF,
FrameMaker and the Adobe PDF printer [set as the Windows default
printer!] will automagically transform any markers such as:
openlink document2.fm
... to get Acrobat to make a link to open "document2.pdf"
Answering your questions in turn:
#1 - Goto URL usage
This is a strange "hypertext" command. If you print your document to
PDF, Acrobat will have a link to 'address' the operating system to open
the specified URL. URLs start with typically http:// or file:// and
the address part always uses forward slashes.
#2 - forward or backward slash
Always use forward slashes in pathnames for FrameMaker document linking.
It does work!
I advise against using backslashes ... as FrameMaker uses the backslash
to specify special characters, such as tab \t - but also, if you edit a
marker, you will ALWAYS have to keep doubling the number of backslashes
in the UNC [Uniform Naming Convention] pathname, as FrameMaker will keep
interpreting them.
#3 - See #2
#4 - The history of FrameMaker also means that there's some quirky
features or methods involved. In the days when Frame Technologies [and
later Adobe] ported FrameMaker to multiple platforms, they standardised
on paths to be Unix style - even for Windows and Macintosh, for example
openlink /working-folder/examples/document2.fm
... to get Acrobat to make a link to open
"/working-folder/examples/document2.pdf"
#5 - FrameMaker's hypertext support
There's a PDF document on the MicroType web site that gives a good
overview:
http://www.microtype.com/resources/FMhypertext.pdf